Theory Of Operation

Fig 1: EVAP System Components

Courtesy of CHRYSLER GROUP, LLC

EVAPORATIVE SYSTEM OVERVIEW: 

The Powertrain Control Module (PCM) monitors the Evaporative Emission System operation. The primary concern being monitored is the integrity of the system against leaks. The operation of the Purge Solenoid and evaporative purge flow is also monitored. This is accomplished using inputs from the ESIM Switch and Fuel Tank Pressure Sensor. The basic strategy used is that in a sealed system, pressure will naturally increase or decrease in relation to temperature. As temperature increases, so does pressure inside the system. And conversely, as temperature decreases, pressure in the system will decrease as well and will eventually turn into a vacuum if no leaks are present. Even the smallest of leaks can be accurately detected in this manner. The normally open vacuum pressure switch within the ESIM will close when vacuum in the evaporative system reaches approximately -0.036 psi. A normally closed vacuum relief valve inside the ESIM is used to maintain the seal on the evaporative system during the engine off condition. If vacuum in the evaporative system exceeds a calibrated threshold, the relief valve will pull off its seat to protect against excessive vacuum conditions which can damage the tank. If pressure inside the evaporative system exceeds a calibrated threshold, a normally closed pressure relief valve inside the ESIM will unseat and relieve the excess pressure, thus limiting any pressure build up in the tank. This is beneficial because the induced vacuum during a subsequent declining temperature will achieve the switch closed (pass threshold) sooner than if the tank had to decay from a higher built up pressure.

ESIM SWITCH STUCK CLOSED MONITOR: 

At ignition off, the state of the ESIM switch is evaluated. If the switch is open, a pass flag is set and PCM will complete power down. If the switch is closed, the PCM will wait a calibrated delay time and open the Purge Solenoid. This should vent the vacuum in the Fuel Tank to atmosphere through the Intake Manifold. If the switch opens, a pass flag is set and the PCM will power down. If the PCM detects that the ESIM switch is still closed after a calibrated time, an error is detected and failure is set at the next engine run cycle.

SMALL LEAK MONITOR EVENT:  This is an accumulative monitor and the data from each valid event is recorded and added to the previously recorded events. The PCM timer records the engine on/drive cycle and engine off time for each small leak monitor event. For an event to be valid the PCM must see; 

1. An engine on/drive cycle for a minimum of 10 minutes.
   NOTE:

   The engine on timer will stop counting after a maximum of 26 minutes.

2. And, when the engine is shut down, an engine off timer starts. There is a 12 minute delay time in which the PCM will ignore ESIM Switch input. The engine off timer period will continue to count until one of the three conditions exist:
   • The engine is started without a switch closure during the event.
   NOTE:

   At the next key on cycle a determination is made as to whether the event was valid and the information is kept.

   • An ESIM Switch closed input is received after the 12 minute delay during the event.
   NOTE:

   If the switch closed input is received, the PCM records that the switch has closed, stores the engine shut down time, then goes to sleep.

   • After a maximum of 1051 minutes without an ESIM Switch closure during the event.

The accumulative monitor will increment and start over after both timers have reached a calibrated threshold (Engine on - 100 minutes and Engine off - 4200 minutes). When the monitor has exceeded the calibrated accumulated engine on and engine off thresholds, the system is evaluated and the accumulated timer starts over.

LEAK SIZE DETERMINATION:  If the PCM did not see an ESIM Switch closed signal during the previous ignition off cycle, and the event was valid, an intrusive leak test is run to determine if a large leak is present on the next cold start.

Immediately after start-up, while the engine is cold, the Purge Solenoid is opened to create a vacuum in the evaporative system to a calibrated vacuum point that is below the ESIM Switch closing point (1.0 in Hg).

• If the switch does not close at all during purging, because of a switch that is stuck open or vacuum cannot be created below 1.0 in Hg within a calibrated time threshold, it is determined to be a general evaporative system failure (P0440).
• If vacuum is created and the switch has closed, the PCM monitors how long it takes for the switch to open. If the switch opens before the maximum calibrated time, it is determined to be a large leak (P0455).
• If vacuum is created and the switch has closed, the PCM monitors how long it takes for the switch to open. If the switch stays closed longer than a maximum calibrated time before opening, it is determined that a large leak is not present and the Small Leak Monitor will continue to run until the accumulative monitor increments. If no ESIM Switch closures were recorded during the entire increment, it is determined that a small leak is present (P0456).

PURGE FLOW MONITOR: 

The Purge Flow Monitor will only run if the Small Leak Monitor recorded a pass on the previous ignition off event and the test data was valid. Because the leak detection diagnostics can only verify that the fuel tank system is sealed while the purge valve is closed, it cannot determine if the purge line between the solenoid and Intake Manifold is pinched or leaking. The Purge Flow Monitor is needed to verify these failure modes. The Purge Flow Monitor works on the premise that as flow through the system increases, so does the pressure drop in the system. The PCM monitors the Fuel Tank Pressure Sensor and looks for increasing vacuum in the Fuel Tank with increasing purge flow. Conversely, it looks for decreasing vacuum in the Fuel Tank with decreasing flow.

• With the engine running and enable conditions met, the non-intrusive purge monitor  looks for a calibrated increase in vacuum in the fuel tank with increased purge flow, referred to as phase 1. If phase 1 passes, the purge monitor looks for a calibrated decrease in vacuum in the Fuel Tank with decreasing purge flow, referred to as phase 2. If phase 2 passes, the purge monitor is complete. If the purge flow monitor fails either phase, or does not complete both phases within a specified time, an intrusive test is initiated to verify the results from the non-intrusive test.
• The intrusive diagnostic  uses the same two phases to analyze the system. However, the intrusive test actuates the Purge Solenoid such that the differences are much more pronounced allowing a more accurate test result. If the PCM detects a failure during the intrusive test, a purge system performance fault is set (P0441).